Within most, if not all, television receivers, sound or audio information present in the received signal is detected and processed at a low power level to provide an audio signal. This low level audio signal is then amplified by one or more audio frequency amplifiers to a voltage and power level sufficient to drive the television receiver speakers.
As with other audio power amplifiers, television audio amplifiers are to the extent possible optimized for power output and linearity. In most circumstances, the needs of such power amplifiers relate directly to full use of the available supply voltage and optimum drive current to the output amplifying stage. In consumer oriented television receivers, however, competitive costs must be maintained and thus unlimited resources are not available to provide substantial power and linearity at the expense of great cost premiums.
As a result, designers of television receiver audio systems generally are required to limit the number and type of audio power amplifiers used within television receivers. One of the most cost effective high performance amplifier structures used in the art is that known commonly as a "bootstrap" amplifier. In such bootstrap amplifiers, stacked output transistor stages are coupled to a bootstrap capacitor which charges during one portion of the audio signal cycle and thereafter discharges during the remaining portion of the signal cycle to supplement the drive current provided to the stacked transistor pair. Thus, the bootstrap capacitor is intended to supply more drive current to the output transistor pair without the use of more expensive additional predriver stages or higher power supply voltage. Bootstrap amplifiers themselves, are subject to certain operational limitations. One of such limitations arises through the loss of drive energy during portions of the signal cycle resulting from bootstrap capacitor discharge into the operating supply source. This loss of drive energy during a portion of the signal cycle results in nonlinearity of high amplitude output signal swings. Faced with this problem, most practitioners in the art attempt to comprise the signal handling capability of the power amplifier against the undesired nonlinearity during large signal swings.
There arises, therefore, a need in the art for evermore improved cost effective power amplifiers for use in television receiver audio systems.